Conventionally, a cell electrode is fabricated by dissolving a binder for battery (hereinafter, briefly referred to as “binder” when appropriate) in a solvent or dispersing it in a dispersion medium to form a binder composition, applying a slurry for battery electrode, which is a mixture of an active material and the binder composition, (hereinafter, briefly referred to as “slurry” when appropriate) to a collector, removing the solvent or the dispersion medium by a drying method and the like, thereby binding particles of the active material to the collector, and simultaneously to each other.
The battery capacity is determined by various factors, such as, the kind and quantity of the active material, the kind and quantity of the electrolyte and the like, but the performance of the binder is also an important factor. If a binder cannot bind a sufficient amount of active material particles to the collector and cannot allow active material particles to be bound to each other, a battery having large capacity cannot be obtained. Furthermore, the battery capacity is reduced since the active material is parted from the collector due to the repetition of charge and discharge.
For the binder, that is, strong binding powers between the collector and the active material and between active material particles (hereinafter, briefly referred to as “binding power” when appropriate), and a good binding durability by which parting of active material from the collector is prevented and by which separation of the active material particles from each other caused by the volume change of the active material particles due to the repetition of charge and discharge (hereinafter, referred to as “binding durability” when appropriate) are required.
As a binder for lithium secondary battery, a polyvinylidene fluoride polymer is widely used in the related industries. Such polyvinylidene fluoride polymer is dissolved in N-methylpyrrolidone and the like to prepare an organic solvent binder composition, and then, an active material is added to the organic solvent binder composition to form a slurry. Next, the slurry is applied to the collector, and dried to obtain an electrode. However, in the case of using this binder, there are some problems in the binding durability such that the active material particles are parted because of the volume change (due to lack of binding power) of the active material due to the repetition of charge and discharge, as well as insufficient binding power between the collector and the active material (for example, Japanese Unexamined Patent Publication No. H6-163031). It is assumed that these problems may happen because the polyvinylidene fluoride polymer surrounds the active material in the form of fibril, and thereby binding power among active material particles becomes strong, but binding power between the collector and the active material is not strong enough. In addition, since polyvinylidene fluoride polymer has poor rubber elasticity, it does not become effectively responsive to the volume change of the active material caused by the repetition of charge and discharge. Thus, sufficient binding durability for preventing the active material particles from being parted from each other cannot be obtained.
Therefore, considering the elasticity of a rubber, it is proposed to use a paste which is a mixture of an active material and an uncrosslinked rubber solution (for example, Japanese Unexamined Patent Publication Nos. H3-53450, H5-62668, and others), but the capacity may be reduced when this binder is used. Also, it is suggested to use rubber particles of a latex in an aqueous medium as a binder (for example, Japanese Unexamined Patent Publication Nos. H5-21068, 5-74461, and others), but the binding power between the collector and the active material of a conventional rubber latex is not so strong enough to reach the expected level.
In the case where such known rubber is used as a binder, a large elasticity-imparting effect can be obtained in terms of the binding durability, but a sufficient effect is not expected in terms of binding powers between an active material and a collector or among active material particles.
A method for intensifying binding power by use of mixing and dissolving carboxymethyl-cellulose in an aqueous styrene-butadiene rubber latex has been suggested (Japanese Unexamined Patent Publication No. H4-342966, etc.). However, since carboxymethyl-cellulose reduces electrode flexibility, the balance of binding power with binding durability of the rubber latex is affected, so that electrode having sufficient performance cannot be obtained. Thus, it is currently required to develop a novel binder for lithium secondary battery having excellent binding power and binding durability as well as excellent battery characteristics.